[PATCH 7/7] Documentation: rust: testing: add docs on the new KUnit `#[test]` tests

Miguel Ojeda
2025-05-02T21:52:20Z

There was no documentation yet on the KUnit-based `#[test]`s.

Thus add it now.

It includes an explanation about the `assert*!` macros being mapped to
KUnit and the support for `-> Result` introduced in these series.

Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
---
Documentation/rust/testing.rst | 71 ++++++++++++++++++++++++++++++++++
1 file changed, 71 insertions(+)

diff --git a/Documentation/rust/testing.rst b/Documentation/rust/testing.rst
index 6337b83815ab..f43cb77bcc69 100644
--- a/Documentation/rust/testing.rst
+++ b/Documentation/rust/testing.rst
@@ -130,6 +130,77 @@ please see:

https://rust.docs.kernel.org/kernel/error/type.Result.html#error-codes-in-c-and-rust

+The ``#[test]`` tests
+---------------------
+
+Additionally, there are the ``#[test]`` tests. Like for documentation tests,
+these are also fairly similar to what you would expect from userspace, and they
+are also mapped to KUnit.
+
+These tests are introduced by the ``kunit_tests`` procedural macro, which takes
+the name of the test suite as an argument.
+
+For instance, assume we want to test the function ``f`` from the documentation
+tests section. We could write, in the same file where we have our function:
+
+.. code-block:: rust
+
+ #[kunit_tests(rust_kernel_mymod)]
+ mod tests {
+ use super::*;
+
+ #[test]
+ fn test_f() {
+ assert_eq!(f(10, 20), 30);
+ }
+ }
+
+And if we run it, the kernel log would look like::
+
+ KTAP version 1
+ # Subtest: rust_kernel_mymod
+ # speed: normal
+ 1..1
+ # test_f.speed: normal
+ ok 1 test_f
+ ok 1 rust_kernel_mymod
+
+Like documentation tests, the ``assert!`` and ``assert_eq!`` macros are mapped
+back to KUnit and do not panic. Similarly, the
+`? <https://doc.rust-lang.org/reference/expressions/operator-expr.html#the-question-mark-operator>`_
+operator is supported, i.e. the test functions may return either nothing (i.e.
+the unit type ``()``) or ``Result`` (i.e. any ``Result<T, E>``). For instance:
+
+.. code-block:: rust
+
+ #[kunit_tests(rust_kernel_mymod)]
+ mod tests {
+ use super::*;
+
+ #[test]
+ fn test_g() -> Result {
+ let x = g()?;
+ assert_eq!(x, 30);
+ Ok(())
+ }
+ }
+
+If we run the test and the call to ``g`` fails, then the kernel log would show::
+
+ KTAP version 1
+ # Subtest: rust_kernel_mymod
+ # speed: normal
+ 1..1
+ # test_g: ASSERTION FAILED at rust/kernel/lib.rs:335
+ Expected is_test_result_ok(test_g()) to be true, but is false
+ # test_g.speed: normal
+ not ok 1 test_g
+ not ok 1 rust_kernel_mymod
+
+If a ``#[test]`` test could be useful as an example for the user, then please
+use a documentation test instead. Even edge cases of an API, e.g. error or
+boundary cases, can be interesting to show in examples.
+
The ``rusttest`` host tests
---------------------------

--
2.49.0